Without limiting the scope of the invention, its background is described in connection with a small pixel CCD image sensor, as an example.
Heretofore, in this field, increasing demand from the consumer market to supply higher and higher resolution CCD cameras has required designers to increase the number of pixels in the CCD image sensors that are used in these products. At the same time the competitive pressures to maintain or reduce the cost of these sensors necessitate the reduction of chip size and consequently the reduction of the active pixel area. This inevitable trend is clearly observed in the recent technical literature and also in the many product catalogs. This, however, leads to the two major problems: reduction in the image sensor sensitivity and reduction in the signal-to-noise ratio.
The sensitivity of the image sensors is proportional to the pixel area, the aperture efficiency, the quantum efficiency, the integration time, and the charge to voltage conversion factor. The maximum sensor sensitivity of present day image sensors is usually determined by the noise floor of the charge detection amplifiers which convert the collected charge into an output voltage. Without this limit it would be possible to detect the individual photons and thus achieve a photon counting operation, the ultimate performance of an image sensor which is determined by the fundamental laws of physics.
To reduce the noise floor of the charge detection amplifiers, however, is not an easy task. To achieve better performance, the sensor designers are developing various schemes of charge-to-voltage conversion using complicated kTC noise suppression circuits ("A New Noise Suppression Method for High-Definition CCD Camera", IEEE Trans. on Consumer Electronics, vol. 35, No. 3, pp. 368-374, August 1989), current modulation techniques ("A 250k-Pixel SIT Image Sensor Operating in its High-Sensitivity Mode", IEEE Trans. on Electron Devices, vol. ED-38, pp.1021-1027, May 1991), or employing many other novel charge detection structures ("New Low-Noise Output Amplifier for High-Definition CCD Image Sensor", IEEE Trans. on Electron Devices, vol. ED-38, pp. 1048-1051, May 1991). There is, however, a well known technique, which has been used for many years in image orthicon camera vacuum tubes. It is a carrier multiplication process.
There are many solid state devices available on the market today that utilize some form of carrier multiplication. These devices are typically referred to as "Avalanche Photo Diodes" (APD) and are used in optical communications. A large amount of literature has been accumulated on this subject and a good review can be found elsewhere ("Lightwave Communication Technology", Semiconductors and Semimetals, vol. 22, New York:Academic Press, 1985). Recently, an article describing the carder multiplication in a sensing pixel that can be used in an area image sensor has also appeared ("A novel High-Gain Image Sensor Cell Based on Si p-n APD in Charge Storage Mode Operation", IEEE Trans. on Electron Devices, vol. ED-37, pp. 1861-1868, August 1990).
The carrier multiplication as is used in the solid state APD sensors and in common vacuum tube photomultipliers is effective in increasing the sensitivity and the signal to noise ratio, since the noise associated with this process is very low. This allows the sensor to increase the number of carriers well above the amplifier noise floor and thus provide the photon counting mode of operation. The signal-to-noise ratio of the output signal is then almost equal to the signal-to-noise ratio of the input photon flux rather than being limited by the noise of the charge detection amplifiers. However, devices based on the APD concept are too large to incorporate into a pixel of an image array and the gains may be limited.
Some of the problems faced by designers have been the inability to incorporate the charge multiplication function into a solid state image sensor array in a way that would allow for higher carrier multiplication factors comparable to vacuum tube photodetectors (which do not record an image ), and also devising a method to achieve higher multiplication factors than are possible with devices employing carrier multiplication in the photosite. Accordingly, improvements which overcome any or all of the problems are presently desirable.